This invention relates to a method of treating Bayer process red mud slurries to improve or facilitate the handling of such slurries.
In the Bayer process for obtaining alumina from bauxite, the bauxite is initially digested with caustic soda to extract alumina values as dissolved sodium aluminate in an aqueous liquor which also contains, in suspension, undissolved particulate residues of the original bauxite. The particulate residues, known as red mud, are separated from the Bayer process liquor and collected in a concentrated slurry for transport to a storage or disposal site, while the process liquor is subjected to decomposition to precipitate alumina trihydrate, the latter being calcined to recover the desired alumina product.
Red mud is generally considered as a waste product, although it has some present or potential uses. Chemically, it comprises, in varying amounts (depending upon the composition of the starting bauxite), oxides of iron and titanium, residual alumina, sodalite, silica, and minor quantities of other metal oxides.
In the Bayer process, after initial separation from the process liquor, the red mud is typically washed in a countercurrent washing circuit to remove most of the caustic soda carried with the red mud from the starting liquor. It is thereby made up into an aqueous slurry, which is thickened or filtered to a high solids concentration (high solids consistency, typically about 10-70% by weight solids) for delivery to a storage or disposal site. The percentage of solids depends on the granulometry of the bauxite and of the red mud. For very finely divided red mud, such as obtained from Jamaica bauxite, the solids content is between 15 and 30%; for coarser bauxite and red mud, such as obtained from African bauxite, the solids content can range from 60 to 70%. The aqueous vehicle of this slurry still has significant alkalinity, e.g. a concentration of NaOH (expressed as Na2CO3) of 1 to 40 g/L, but more usually from 4 to 25 g/L. Such high-solids, alkaline red mud slurries underflow the mud thickeners and mud filters of Bayer process plants, and are usually pumped therefrom (ordinarily with high pressure positive displacement pumps) over relatively long distances to storage or disposal sites. The term "Bayer process red mud slurry" will be used herein to refer to alkaline aqueous red mud slurries of this character, viz., slurries of red mud as separated from a Bayer process line and prepared for discharge or transport to a storage/disposal site.
One known technique for disposing of Bayer process red mud slurry is known as "stacking" or "stacking and drying." This technique involves adjusting the solids concentration of the slurry to produce a flowability that will enable the slurry to stack, at its angle of repose, at the disposal site. The angle of repose needs to be within fairly well defined limits in order to allow rainfall to drain off the stack rapidly, but not so rapidly as to create erosion channels in the stack, and to maximize the life of the stacking site.
Heretofore, the handling and pumping of Bayer process red mud slurries have presented problems, owing in particular to the high viscosity (and consequent low flowability for transport) of such slurries. To facilitate pumping and reduce pumping pressure or to obtain a lower consistency of the red mud slurry, water is often added. The addition of water, however, is attended with disadvantages, in that it is a diluent which must be removed (as by evaporation and/or drainage) to achieve proper consolidation of the mud at the disposal site. The necessity for such removal slows down the consolidation of the mud. Thus, in the stacking and drying technique, stacking is followed by solar drying of the stack to produce a dry, hard solid. In this case it is important to minimize, or eliminate completely, the addition of water to increase flowability, as such addition increases the evaporation load at the disposal site. Furthermore, because of the unavoidable alkalinity of Bayer process red mud slurries, the water drained at the disposal site cannot be simply discharged to the environment but must be pumped back to the plant.
Various expedients that have been proposed to increase the flowability of Bayer process red mud slurries without addition of water have included violent agitation or the addition of a dispersant. Agitation, however, often has only limited effectiveness, and the addition of dispersants is frequently expensive (in cost per unit of tailing solids, i.e. red mud, treated). As illustrative of such dispersants, U.S. Pat. No. 4,146,573 describes adding a low molecular weight polymer such as acrylic acid polymer to enhance the fluidity of the mud, and U.S. Pat. No. 4,464,479 describes admixing the red mud (e.g. directly after the red mud has been separated by filtration from the Bayer process liquor) with coal dust, preferably very finely divided brown coal or lignite, in an amount equal to 10%-200% of the red mud, so as to reduce flowability and produce a grainy mixture which can be easily handled or transported.
Additives have also been proposed to improve the consistency red mud as a useful commercial product. Thus, U.S. Pat. No. 3,886,244 sets forth that the consistency of red mud when used to make bricks can be adjusted by adding a water-fixing substance such as coal dust, cork or peat.
Humic substances are naturally occurring complex organic materials always found in low to medium concentrations in soils, but which occur in deposits of high concentration in some geographic areas. Humic substances have been described by Schnitzer, M., and Khan, S. U., on page 3 of Humic Substances in the Environment. New York: Marcel Dekker Inc., 1972, as amorphous brown or black, hydrophilic, acidic, polydisperse substances of molecular weight from several hundred to tens of thousands. Humic substances are generally classified according to their solubility in alkali and acid as follows:
(1) Humic Acid: soluble in dilute alkali but is precipitated on acidification of the alkaline extract.
(2) Fulvic Acid: humic fraction that remains in the aqueous acidified solution, i.e. soluble in both acid and alkali.
(3) Humin: humic fraction that cannot be extracted by dilute alkali and acid.
High concentrations of humic acids can be found in lignite or brown coal. According to Kirk-Othmer Encyclopedia of Chemical Technology, 3d ed., vol. 14, pp. 313-43, lignite and brown coal are terms used to describe coals of the lowest degree of coalification. Typically, lignite deposits are from one to 60 million years old. Lignitic coals tend to oxidize more readily than older ones. The moisture content of lignites is much higher than that of older coals, ranging up to 70%. The oxygen:carbon ratio ranges above 0.2. Low temperature carbonization produces oils for conversion to motor fuels.
Leonardite is a naturally oxidized form of lignite (brown coal) with a high content of humic acids, which may be described as carboxylated phenolic polymers. It is described in Kirk-Othmer Encyclopedia of Chemical Technology, 3d ed., vol. 17, p. 155 (1982), and in U.S Pat. No. 4,778,608. Leonardite is mined, in the western United States, e.g., in Williston, North Dakota, and manufactured into dry, free flowing powders. It has been used in drilling fluids as a thinner (to adjust the flowability of drilling muds), for filtration reduction and for oil emulsification and as a stabilizer, and to reduce the viscosity of coal slurries pumped in long-distance pipelines.
There is no sharp demarcation line between lignite and leonardite. It is to be understood that the terms "lignite" and "leonardite" are used herein interchangeably to denote a naturally occurring material that contains useful concentrations of humic acid or humates. Furthermore, the term "humates" is used herein to mean the salts of humic acids, which either occur in nature or are formed when humic acids react with substances which convert the acids to another form.